This invention relates generally to semiconductor lasers and, more particularly, to semiconductor laser structures providing relatively high output powers. There are a number of applications of semiconductor lasers that require relatively high powers, such as space communications, laser printing, and optical recording. In recent years, much of the development effort in semiconductor lasers has been directed to increasing the power output from lasers in continuous wave (cw) operation.
Basically, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes combine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers to confine the light in the plane of the active layer. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in the plane of the active layer.
Various approaches have been used to confine the light in a lateral sense within a semiconductor laser, i.e. perpendicular to the direction of the emitted light within the plane of the active layer. If a narrow electrical contact is employed to supply current to the device, the lasing action will be limited to a correspondingly narrow region, in a process generally referred to as gain guiding. At high powers, gain-guided devices have strong instabilities and produce highly astigmatic, double-peaked beams. For most high-power semiconductor laser applications there is also a requirement for a diffraction-limited beam, i.e. one whose power is limited only by the diffraction of light, to a value roughly proportional to the wavelength of the emitted light divided by the width of the emitting source. Because of the requirement for a diffraction-limited beam, most research in the area has been directed to index-guided lasers. In these, various geometries are employed to introduce dielectric waveguide structures for confining the laser light in a lateral direction, perpendicular to the direction of light emission and generally in the same plane as the active layer.
A useful introduction to these and other considerations in the design of semiconductor lasers can be found in a paper by Dan Botez, entitled "Laser diodes are power-packed," IEEE Spectrum, June, 1985, pp. 43-53. One highly promising technique for achieving high powers and good lateral index-guiding involves the formation of a longitudinal channel in a semiconductor substrate. The substrate, including the channel, is covered with a first cladding layer, then the active layer and the other cladding layer. This channeled substrate planar (CSP) structure provides a high degree of optical absorption to the substrate on both sides of the channel, and therefore discriminates against higher-order modes of oscillation. The CSP laser structure can be fabricated most easily using the liquid-phase epitaxial (LPE) process, but this has practical limitations and should be avoided. First the LPE process is limited in the size of semiconductor wafer that can be handled, so that mass production of the device is limited. Moreover, the LPE process is extremely time-consuming, such that only a single production run may be completed in a typical working day.
In brief, although the LPE process lends itself well to the filling of the channel region, to facilitate the use of a planar active layer, the process has practical limitations, and it would be highly desirable to develop a more attractive alternative approach. The process of metalorganic chemical vapor deposition (MOCVD) can be employed on larger semiconductor wafers and has other advantages, such as a production run time of about two hours, which permits three or four runs per working day. However, MOCVD is unable to form a CSP structure without difficulty, since it is incapable of completely filling the substrate channel, and a non-planar active layer results. In addition, there is usually difficulty in growing the first cladding layer onto the substrate in the channel region. If the material is gallium aluminum arsenide (GaAlAs), and surface oxidation is not totally removed from the substrate surface, the device may have a substantially high threshold current.
Accordingly, there is a need for a semiconductor laser structure with the functional performance of a channeled substrate planar structure, but with the ability to be fabricated with a process other than liquid-phase epitaxy. The present invention is directed to this end.